Add new security-policy documentation.

This formally describes our policies on reporting, notification and
disclosure of security issues, and provides a detailed explanation of
our full security-response process, for reference purposes.

This is necessary for the 'utils' tests to pass.

Based on Vinay Sajip's branch.

* Renamed smart_unicode to smart_text (but kept the old name under
  Python 2 for backwards compatibility).
* Renamed smart_str to smart_bytes.
* Re-introduced smart_str as an alias for smart_text under Python 3
  and smart_bytes under Python 2 (which is backwards compatible).
  Thus smart_str always returns a str objects.
* Used the new smart_str in a few places where both Python 2 and 3
  want a str.

Grammar fix in 1.5 release notes.

I believe this is standard.

Thanks shelldweller for the draft patch.

Fixed #15754 -- avoid recursively computing the tree of media widgets more times than is necessary for a wiget

Updated example of customized ModelAdmin in documentation for 1.4

Moved a note about django-admin.py errors from Tutorial Part 1 to a new FAQ Troubleshooting page. This is to avoid confusion for beginners.

Thanks Raman Barkholenka for the patch.

Ticket 18632 Cleanup of CBV edit/edit mixin documentation.

Fixed #18110 -- Improve template cache tag documentation

Fixed small typo in class based view docs.

Thanks Pratyush for the report.